A bar code symbol is a pattern of parallel bars and spaces of various widths that represent data elements or characters. The bars represent strings of binary ones and the spaces represent strings of binary zeros. A conventional “one-dimensional” bar code symbol contains a series of bars and spaces that vary only in a single dimension. One-dimensional bar code symbols have relatively small information storage capacities. “Two-dimensional” bar codes have been developed to meet the increasing need for machine-readable symbols that contain more information than one-dimensional bar code symbols. The information storage capacity of two-dimensional bar code symbols is increased relative to one-dimensional bar codes by varying a series of bars and spaces in two dimensions. Common two-dimensional bar code standards include PDF417, Code 1, and Maxicode. One-dimensional and two-dimensional bar code symbols typically are read by optical scanning techniques (e.g., by mechanically scanned laser beams or by self-scanning charge-coupled devices (CCD's)) that convert a printed bar code symbol into electrical signals. The electrical signals are digitized and decoded to recover the data encoded in the printed bar code symbol.
Bar code symbols may be used in a variety of applications, including low information content applications (e.g., automatic price tagging and inventory management), and relatively high information content applications (e.g., encoding mail addresses and postage for automated mail reading and mail distribution systems, and encoding compressed content of a printed page).
The United States Postal Service (USPS) has proposed an Information Based Indicia Program (IBIP) that allows users to purchase and print postage with their computers. In accordance with the IBIP, the printed postage indicium includes a large, two-dimensional bar code that contains certain required postal data, including information that conveys evidence that postage has been paid and information used for mail processing. The required postal data is cryptographically transformed into a digital token using a cryptographic key that is held within a secure postage accounting device that is assigned to each user. The digital token is validated by a verifier with access to a public key corresponding to the secret key of the accounting device. By validating the information embedded within the postage indicia, the USPS may verify the integrity and authenticity of the data contained within the postage indicia and, therefore, may be reasonably certain that the required postage has been paid.